Melt-spinnable, aromatic polyesters prepared from aromatic diols and mono-substituted terephthalic acids wherein the substituents include Br, Cl, alkyl of 1-3 carbon atoms and alkoxyl are known (U.S. Pat. Nos. 4,065,432; 4,156,070; U.K. Pat. No. 993,272). Polyesters containing phenyl-substituted isophthalic acid are disclosed in U.K. Pat. No. 885,739.
Melt-spinnable, fiber-forming polyesters capable of forming optically anisotropic melts, prepared from monophenylhydroquinone and terephthalic acid, are disclosed in U.S. Pat. No. 4,159,365.
Phenylterephthalic acid is known (Annalen 586, 138-46 (1954)).
The art is confusing on the relative effects of substituents on the diacid and diol components of aromatic polyesters. U.S. Pat. No. 4,118,372 teaches that, in the preparation of aromatic, melt-spinnable polyesters capable of forming optically anisotropic melts, it is preferable that the diol be substituted with halogen or lower alkyl and that the diacid remain unsubstituted because of thermal or hydrolytic instability and/or cost of copolymers prepared from ring-substituted aromatic diacids. U.S. Pat. No. 3,160,603 teaches that aromatic, fiber-forming, initially amorphous polyesters in which the diacid is chloro-substituted show much less tendency to crystallize than isomeric polyesters wherein the diol is chloro-substituted. U.K. Pat. No. 993,272 discloses unexpectedly crystalline polyesters prepared from (asymmetric) monoethyl-, alkoxyl- or chloro-substituted terephthalic acid and like-substituted hydroquinone. A substantial amount of an unsubstituted third reactant (diacid or diol) may be incorporated while retaining crystallinity.
U.S. Pat. No. 4,118,372, supra, also discloses aromatic melt-spinnable copolyesters, capable of forming optically anisotropic melts, which contain recurring units of (O-R.sub.1 -O), (OC-R.sub.2 -CO), and (O-R.sub.3 -CO) wherein R.sub.1, R.sub.2, and R.sub.3 are arylene radicals, including 1,4-phenylene, 4,4'-biphenylene and 2,6-naphthylene, and R.sub.1 and R.sub.3 can contain one or more substituents, including lower alkyl of 1-4 carbon atoms.
U.S. Pat. No. 4,159,365, supra, also discloses that polyesters containing units derived from monophenylhydroquinone and terephthalic acid may also contain units derived from 4-hydroxybenzoic acid, 4,4'-dihydroxybiphenyl and 1,4-hydroquinone.
U.S. Pat. No. 4,242,496 and the related PCT Application Publication Number W079/01034 disclose melt-spinnable, aromatic, liquid crystal (anisotropic melt) copolyesters of terephthalic acid, phenylhydroquinone and p-hydroxybenzoic acid (HBA). The polyesters contain 25 to 80 mole percent HBA, based on total moles of diacid and hydroxy acid, and exhibit melting points which decrease with increasing HBA content, the melting point reaching a minimum at about 20 mole percent HBA, then rising again with increasing HBA content. Minimum softening temperatures occur at 60 to 70 mole percent HBA.